Science Forums 2

[mynah]

Even as more and more of the mechanisms involved in the evolution of new species are becoming clear, the one that puzzles me is differences in chromosome numbers and their contribution to evolution. Sure, the mechanisms (translocations, inversions, fusions between acrocentric chromosomes, etc.) have been known for a long time, as has the fact that not all human individuals (to mention one species) have the same number of chromosomes, and that differences in chromosome number is a powerful genetic isolating factor.

What I find puzzling is that changes in chromosome number accompanied by speciation is so common. The mechanisms involved are likely to be deleterious, or at best be neutral to the carrier – but even an asymptomatic individual with a translocation or an unusual number of chromosomes has a greatly increased risk of reduced fertility and/or reduced fitness of offspring. As a result, even a rare carrier who benefits personally from a translocation or inversion is likely to have fewer healthy offspring than one who does not have the chromosomal defect.

Of course, if the condition bestows some selective advantage and if there are enough other carriers around for matings to occur frequently between carriers, chromosome numbers may change, and descendants may become genetically isolated from other members of the species. If selection pressure is sufficient, the change will eventually become homozygous in the population.

The question is, however - how is this point reached, and why has it happened so often during the course of evolution?